1. Field of the Invention
The present invention relates to a method and apparatus for monitoring the fiber density of polymeric composite materials during fabrication, and more particularly to a non-invasive method and apparatus that can be used during the fabrication process.
2. Description of the Prior Art
As will be appreciated by those skilled in the art, composite polymeric materials comprise multiple layers of a fiber material, such as a carbon fiber material, impregnated with a polymer, usually a thermosetting polymer. The layers of material and the thermosetting polymer are fabricated into a solid piece by the simultaneous application of heat and pressure. Certain important physical characteristics of the fabricated piece, such as its sheer strength, are a function if its fiber density.
There have been proposals in the prior art relating to monitoring a thermosetting polymer composite during the fabrication process. U.S. Pat. No. 4,891,591 to Johnston et al., discloses a nonintrusive method of monitoring the cure (i.e., extent of reaction) of a polymeric material using an electromagnetic field to sense a change of resistance of the polymeric material in the electromagnetic field that occurs during curing. This change of resistance is used to vary the impedance of an alternating voltage power supply that produces the electromagnetic field and which change of impedance is measured periodically or continuously to monitor the cure of said polymeric material. Apparatus for practicing the method of that invention may include a nonintrusive sensing head providing an inner, electromagnetic core within an open ended outer pot formed of magnet material, the open end of the pot core being positioned from a selected area of the surface of a sheet of the polymeric material. An alternating voltage supply circuit that includes an inductance coil around said electromagnetic core and a capacitor connected in parallel with said inductance coil to form a resonant tank circuit when energized. The resulting change in resistance of the polymeric material opposite the open end (a function of the curing of the material), is measured as a corresponding change in the impedance of said power supply circuit to thereby monitor the curing of the polymeric material in the selected area.